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.TH MQ_NOTIFY 3C "Feb 5, 2008"
.SH NAME
mq_notify \- notify process (or thread) that a message is available on a queue
.SH SYNOPSIS
.LP
.nf
#include <mqueue.h>

\fBint\fR \fBmq_notify\fR(\fBmqd_t\fR \fImqdes\fR, \fBconst struct sigevent *\fR\fInotification\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBmq_notify()\fR function provides an asynchronous mechanism for processes
to receive notice that messages are available in a message queue, rather than
synchronously blocking (waiting) in \fBmq_receive\fR(3C).
.sp
.LP
If \fInotification\fR is not \fINULL\fR, this function registers the calling
process to be notified of message arrival at an empty message queue associated
with the message queue descriptor, \fImqdes\fR. The notification specified by
\fInotification\fR will be sent to the process when the message queue
transitions from empty to non-empty. See \fBsignal.h\fR(3HEAD). At any time,
only one process may be registered for notification by a specific message
queue. If the calling process or any other process has already registered for
notification of message arrival at the specified message queue, subsequent
attempts to register for that message queue will fail.
.sp
.LP
If \fInotification\fR is \fINULL\fR and the process is currently registered for
notification by the specified message queue, the existing registration is
removed. The message queue is then available for future registration.
.sp
.LP
When the notification is sent to the registered process, its registration is
removed. The message queue is then available for registration.
.sp
.LP
If a process has registered for notification of message arrival at a message
queue and some processes is blocked in \fBmq_receive\fR(3C) waiting to receive
a message when a message arrives at the queue, the arriving message will be
received by the appropriate \fBmq_receive\fR(3C), and no notification will be
sent to the registered process. The resulting behavior is as if the message
queue remains empty, and this notification will not be sent until the next
arrival of a message at this queue.
.sp
.LP
Any notification registration is removed if the calling process either closes
the message queue or exits.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBmq_notify()\fR returns \fB0\fR; otherwise, it
returns \fB\(mi1\fR and sets \fBerrno\fR to indicate the error.
.SH ERRORS
.sp
.LP
The \fBmq_notify()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 9n
The \fImqdes\fR argument is not a valid message queue descriptor.
.RE

.sp
.ne 2
.na
\fB\fBEBUSY\fR\fR
.ad
.RS 9n
A process is already registered for notification by the message queue.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
_
Standard	See \fBstandards\fR(7).
.TE

.SH SEE ALSO
.sp
.LP
.BR mq_close (3C),
.BR mq_open (3C),
.BR mq_receive (3C),
.BR mq_send (3C),
.BR mqueue.h (3HEAD),
.BR siginfo.h (3HEAD),
.BR signal.h (3HEAD),
.BR attributes (7),
.BR standards (7)
